In recent years, a “wireless sensor network” has been proposed, in which sensor nodes including wireless communication units and sensors are arranged in an observation area and pieces of sensing information are collected with a “collection period (i.e., sampling period of sensing values”. For example, each of sensor nodes in a wireless sensor network includes an energy harvesting unit that generates electricity by converting environmental energy (for example, sunlight, vibrations, or the like) into electrical energy, and includes a battery that stores the electricity generated by the energy harvesting unit. Each of the sensor nodes operates by using the electricity stored in the battery.
Furthermore, a sensing value measured by each of the sensor nodes in the wireless sensor network is reported to a “control device” (i.e., network device)” via a gateway device. Here, the wireless sensor network has a tree topology, for example. Specifically, the wireless sensor network includes a sensor node in a “relay operation state” (i.e., a “relay node”) for relaying transmission of sensing values measured by other sensor nodes, and a sensor node in a “non-relay operation state” (i.e., an “end node”). The sensing value measured by each of the sensor nodes is transmitted from each of the sensor nodes to the gateway device directly or via one or more relay nodes. That is, the wireless sensor network includes a “multi-hop communication path”.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-271231
Incidentally, the sensing values vary among the sensors due to various causes. For example, firstly, the sensing values vary due to individual differences among the sensors (a first cause of variation). Furthermore, secondly, the sensing values vary due to individual differences among devices equipped with the sensors (manufacturing precision or the like) (a second cause of variation). Thirdly, the sensing values vary depending on the environment in which the sensors are installed (for example, vibrations, wind, rain, mischief, or the like) (a third cause of variation).
Meanwhile, for example, with respect to the first cause of variation and the second cause of variation, it may be possible to take a countermeasure such as performing calibration before shipping a sensor node. Furthermore, with respect to the third cause of variation, it may be possible to take a countermeasure such as designing a housing of a sensor node so as not to be influenced by an external factor.
However, each of the countermeasures has a problem such as an increase in the load of a calibration process or an increase in the cost for the countermeasures.